1. Field of the Invention
The present invention relates generally to communications systems and particularly to power control in a code division multiple access communication system.
2. Description of Related Art
Because the radio frequency (RF) spectrum is limited, the government, more particularly, the Federal Communications Commission (FCC), governs the use of the radio frequency spectrum. This regulation includes deciding frequency band allocation among the various industries. Since the RF spectrum is limited, only a small portion of the spectrum can be assigned to each industry. Accordingly, the assigned spectrums must be used efficiently in order to allow as many frequency users as possible to have access to the spectrum.
Because the number and size of frequency bands are limited, multiple access modulation techniques are continuously being developed and improved to improve efficiency and capacity and to maximize use of the allocated RF spectrum. Examples of such modulation techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
CDMA modulation employs a spread spectrum technique for the transmission of information. CDMA modulation techniques are becoming popular because they enable more users to communicate at a given time. A spread spectrum system uses a modulation technique that distributes the transmitted signal over a wide frequency band. This frequency band is typically substantially wider than the minimum bandwidth required for transmitting the signal. The spread spectrum technique is accomplished by modulating each baseband data signal to be transmitted with a unique wideband spreading code. Using this technique a signal having a bandwidth of only a few kilohertz can be spread over a bandwidth of more than a megahertz. A form of frequency diversity is obtained by spreading the transmitted signal over a wide frequency range. Since only 200-300 kHz of a signal is typically affected by a frequency selective fade, the remaining spectrum of the transmitted signal is unaffected. A receiver that receives the spread spectrum signal, therefore, will be affected less by the fade condition. In addition, spreading the signals over a large bandwidth allows system robustness against frequency selective interference, as the effect of the de-spreading process is to effectively dissipate the interference power over the entire bandwidth.
In a CDMA telephone system, multiple signals are transmitted at the same frequency. A particular receiver then determines which signal is intended for that receiver by the unique spreading code in the signal. The signals at that frequency without the particular spreading code intended for that particular receiver appear as noise to the receiver and are ignored. Because of this, it is desirable in CDMA systems to transmit at a minimum power level. Thus, CDMA systems typically employ power control algorithms to reduce the power transmission levels. By minimizing power transmission levels, interference to other signals is reduced and network capacity is maximized.
Frame erasures that occur during the data transfers are handled by a combination of power control algorithms that are applicable for packet data of any type. The power control algorithm(s) ensures that the transmission power is sufficient for the current channel conditions to meet a specified frame error rate target.
One problem that has been encountered with respect to the power control algorithms is the synchronization of the forward gain amount of a plurality of BTS's that are in simultaneous communication with a mobile station. As is known, one code division multiple access (CDMA) type of mobile station frequently communicates with a plurality of base station transceiver systems (BTS's) at once and sums the received signals to obtain a signal having adequate signal strength for processing and interpretation. Ideally, the signal strength received from each of the BTS's will be nearly equal. However, because current schemes do not provide for individualized power control commands from the mobile station to the base stations, the mobile station transmits one set of power control commands that are broadcast to all of the BTS's in communication with the mobile station at that time (the BTS's that are listed in the mobile station's active set). Unfortunately, erasures of power control signals will sometimes occur for signals resulting in a situation in which only some of the BTS's receive and respond to the power control signals. Accordingly, the forward transmission power levels, or forward gain, of the BTS's in communication with the mobile station go out of synchronization meaning that they become unequal.
The above problem is generally considered to be a difficult problem without a perfect solution under the current interfaces and standards. One solution has been to ignore the problem altogether by not attempting to synchronize the forward-link gains of the BTS's at all. Accordingly, for those approaches, narrow limits on the forward-link gain values are set on the BTS's so as to limit the amount of divergence that can occur. One problem with this approach, however, is that it limits the actual dynamic range of the forward-link gain available to the BTS's. Thus, limiting such gain limits performance. Another suggested approach is to have a base station controller periodically generate forward gain power transmission levels to each of the BTS's to synchronize them. While this approach is advantageous in that it results in synchronized BTS operation, it is problematic in that power control commands received from the mobile station are overwritten or ignored. Accordingly, a new forward gain power transmission value, as dictated by BSC, may be either higher or lower than is required for the mobile station to adequately receive the communication signals. In the scenario where the power transmission level is higher, the result is unacceptable because of the well known goal in CDMA systems to minimize power transmission levels so as to maximize through put capacity. On the other hand, if the commanded forward gain power transmission level is lower than is required by the mobile station, then the mobile station stands to lose data and to have to repeat its process of increasing the power transmission levels.
What is needed, therefore, is a method for more effectively synchronizing the forward gain power transmission levels of the plurality of BTS's in communication with a mobile station while respecting the power transmission level increases requested by the mobile station in its execution of its power control algorithms.